Saving NSBitmapImageRep as image - objective-c

Question

I've done an exhaustive search on this and I know similar questions have been posted before about NSBitmapImageRep, but none of them seem specific to what I'm trying to do which is simply:
Read in an image from the desktop (but NOT display it)
Create an NSBitmap representation of that image
Iterate through the pixels to change some colours
Save the modified bitmap representation as a separate file
Since I've never worked with bitmaps before I thought I'd just try to create and save one first, and worry about modifying pixels later. That seemed really straightforward, but I just can't get it to work. Apart from the file saving aspect, most of the code is borrowed from another answer found on StackOverflow and shown below:
-(void)processBitmapImage:(NSString*)aFilepath
{
NSImage *theImage = [[NSImage alloc] initWithContentsOfFile:aFilepath];
if (theImage)
{
CGImageRef CGImage = [theImage CGImageForProposedRect:nil context:nil hints:nil];
NSBitmapImageRep *imageRep = [[NSBitmapImageRep alloc] initWithCGImage:CGImage];
NSInteger width = [imageRep pixelsWide];
NSInteger height = [imageRep pixelsHigh];
long rowBytes = [imageRep bytesPerRow];
// above matches the original size indicating NSBitmapImageRep was created successfully
printf("WIDE pix = %ld\n", width);
printf("HIGH pix = %ld\n", height);
printf("Row bytes = %ld\n", rowBytes);
// We'll worry about this part later...
/*
unsigned char* pixels = [imageRep bitmapData];
int row, col;
for (row=0; row < height; row++)
{
// etc ...
for (col=0; col < width; col++)
{
// etc...
}
}
*/
// So, let's see if we can just SAVE the (unmodified) bitmap first ...
NSData *pngData = [imageRep representationUsingType: NSPNGFileType properties: nil];
NSString *destinationStr = [self pathForDataFile];
BOOL returnVal = [pngData writeToFile:destinationStr atomically: NO];
NSLog(#"did we succeed?:%#", (returnVal ? #"YES": #"NO")); // the writeToFile call FAILS!
[imageRep release];
}
[theImage release];
}
While I like this code for its simplicity, another potential issue down the road might be that Apple docs advise us treat bitmaps returned with 'initWithCGImage' as read-only objects…
Can anyone please tell me where I'm going wrong with this code, and how I could modify it to work. While the overall concept looks okay to my non-expert eye, I suspect I'm making a dumb mistake and overlooking something quite basic. Thanks in advance :-)

Answer 1

That's a fairly roundabout way to create the NSBitmapImageRep. Try creating it like this:
NSBitmapImageRep* imageRep = [NSBitmapImageRep imageRepWithContentsOfFile:aFilepath];
Of course, the above does not give you ownership of the image rep object, so don't release it at the end.